FACTORS INFLUENCING BIOLOGICAL RHYTHMS The Circadian Clock Environment

FACTORS INFLUENCING BIOLOGICAL RHYTHMS §The Circadian Clock §Environment –Light, Temperature §Social factors

Factors influencing biological rhythms 1. The Circadian Clock §circadian cycles are controlled a cluster of nerve cells in the hypothalamus in the brain. §The circadian clock's primary function is to interpret external changes of light and darkness. §During night time the nerve cells in hypothalamus sends out the hormone melatonin, which induces sleep. §During winter months there are fewer daylight hours, as a result the level of melatonin secretion increases , thereby increases sleep

2. Environment: Light ØLight and darkness plays an important role in circadian and circannual rhythm ØLight facilitate diurnal animals but inhibits nocturnal animals ØBright light inhibits the rhythm of many protozoans and mammals

Aschoff’s rule • Circadian cycles are influenced by light intensity • Diurnal animals show decreased period length in brighter light • But nocturnal animals shows an increased period of length in brighter light • This generalisation is called Aschoff’s rule

3. Environment: Temperature ØCircadian cycles of cold-blooded /poikilotherms animals are influenced by temperature ØUnder low temperature their rhythm is affected ØEg: snakes, lizards etc find shelter in winter and be active during summer ØWarm-blooded/homeotherm animals are also affected by temperature

3. Social factors: ØSocial stimulation affect reproduction timing in different species ØEg-Anole lizard prepare for reproduction just after their emergence in winter dewlap

An experiment showing influence of social factors §Female lizards were brought from their winter hiding place to lab and grouped into four §Then placed in separate cages §Cage 1 - without males §Cage 2 - castrated /sterile males §Cage 3 - dewlap removed surgically §Cage 4 - normal males §Only 4 th group faced display of colourful dewlap by males and their ovaries developed earlier than other groups §Here visual stimulation plays an important role

AUDITORY/OLFACTORY STIMULATION ØEg-2: Reproductive development of some birds is accelerated by acoustic stimuli ØRoar of male deer affect the onset of estrus in female red deer ØSexual maturity of young female mouse is faster in the presence of scent of the dominant male

ADAPTIVE SIGNIFICANCE OF BIOLOGICAL RHYTHMS 1. Escape from predators 2. Reduces energy demand 3. Reduces competition 4. Increased rate of survival 5. For reproduction

Adaptive significance of biological rhythms 1. Escape from predators: Intertidal crabs exhibit cyclic variation in their shell colour Lighter during daytime and darker during night. 2. Reduces energy demand: Black-capped chicadees decreases their body temperature during night. It helps them to reduce their energy demand 3. Reduces competition: Fly catching birds and insect bats feed on insects. But bats are nocturnal and birds are diurnal so that there is less chance of competition

Adaptive significance of biological rhythms 4. For survival: üFilarial parasite live in the lymph vessel of man üTheir infective stage-microfilaria move to the blood vessel below the skin during night üWhen mosquito suck this blood the parasites reach the body of mosquito üWithout this mosquito the parasite cannot spread and the survival of this parasite depends on mosquito

Adaptive significance of biological rhythms 5. For reproduction: üReproduction time in most birds are spring or rainy season because food is plenty at that time üThis food meet the high energy demands in reproductive activities üHibernation and aestivation also helps them to escape from environmental conditions

o. Environment independent component enables the individual to adjust its activity without any constant check with the environment o. Environment dependent component allows the individual to tune its activity according to the variation in the environment o. Eg: Wild bird sing sharply at 5’o clock in the evening

Human Rhythm Ø Certain activities of human beings are cyclical in nature and such physiological activities of human beings are called human rhythms. ØA very good example for human rhythm is the sleeping and awakening. ØMenstrual cycle is another human rhythm.

Biological Clock § Biological clock is an internal timing device present in all living organisms. §It is an invisible clock existing in every organism such as unicellular protozoans, plants and animals. §It can run and continue even in the absence of environmental cues

PROPERTIES OF BIOLOGICAL CLOCK ØSelf-sustaining timing mechanism ØClock is innate or inherited ØClocks are not regulated by any bio-chemical reaction ØBioclock may run fast or slow but they are reset by environmental cues called entrainers ØHave several clocks to regulate many activities but all these are controlled by a master clock. Eg. Birds undergo reproduction, migration, and so on each controled by seperte clock but are under a master clock

Entrainment of biological clock ØRhythms of many activities do not exactly match with the environment ØExposure of activities to the environment cue can reset the clock and can keep the behavior properly with the environment. This process is called entrainment ØThe environmental cue responsible for entrainment is called entrainer or zeitgebers

Examples to prove the controlling activity of SCN ØSurgical destruction of SCN in hamsters and rats result in loss in circadian rhythmlocomotory cycle, feeding behavior, hormone secretion etc ØIf SCN cells are transplanted they regained this rhythm to some extent.

What constitutes an animal’s biological clock? SCN- Supra Chiasmatic Nucleus ØHypothalamus of brain controls many activities. ØA specific organ of hypothalamus named SCNSupra Chiasmatic Nucleus is considered to perform circadian clock ØSCN is a cluster of cells which receive direct and indirect inputs from nerve fibres originating in retina ØBecause SCN receives information about day and night length which can be used to control the clock

What constitutes an animal’s biological clock? SCN- Supra Chiasmatic Nucleus ØPineal glands inturn controls biological rhythm ØThis gland secretes a hormonemelatonin ØSCN suppresses melatonin synthesis during day time

Chronobiology The formal study of biological temporal rhythms, such as daily, tidal, annual weekly, seasonal, rhythms, chronobiology. is and called